DESCRIPTION: (Investigator's abstract) Protein Z is a vitamin K-dependent plasma protein of previously unknown function. The goals of this proposal are to biochemically characterize: 1) the interaction between protein Z and factor Xa at the surface of phospholipids; 2) the protein mediated enhancement of factor Xa inhibition by tissue factor pathway inhibitor; and 3) the protein Z-dependent inhibition of factor Xa produced by an additional, previously unrecognized plasma protein. In additional studies, the in vivo consequences of protein Z deficiency will be determined in mice. The experiments are designed to provide information, now lacking, concerning the function of protein Z. These results should enhance our understanding of both hemostasis and pathological thromboembolism.